JP2016009614A - Positive electrode active substance material and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active substance material and a lithium ion secondary battery, exhibiting favorable cycle characteristics and having favorable output characteristics.SOLUTION: A positive electrode active substance material includes a positive electrode active substance, a coating layer coating the surface of the positive electrode active substance, and an element diffusion layer at an interface between the coating layer and the positive electrode active substance. The coating layer comprises a lithium-containing polyanion compound, and a polyanion skeleton forming the polyanion compound has a structure with transition metal as the center and has the composition of MO(where, M=V, Cr, Nb, Mo, W; x=0,1; and y=2, 3, 4).

Description

  The present invention relates to a positive electrode active material and a lithium ion secondary battery.

  In recent years, increasing the energy density of lithium ion secondary batteries for mobile devices has been an issue, and increasing the charging potential and simultaneously increasing the capacity and voltage are being investigated as one of the solutions.

However, since the increase in the charging potential promotes the decomposition reaction between the positive electrode active material and the organic electrolyte, there is a problem that the cycle characteristics deteriorate.

  Recently, there is an example in which a reaction with a solid electrolyte is suppressed by coating the surface of a positive electrode active material with a polyanion compound such as lithium-containing phosphate or borate in an all-solid battery (Patent Document 1). In No. 1, phosphorus and boron in the polyanion skeleton are not easily dissolved in the positive electrode active material, and it is difficult to form a stable interface, so that the suppression effect is not sufficient, and lithium-containing phosphates and borates have low electron conductivity. There is a problem that the output characteristics are inferior.

JP 2012-99323 A

    The present invention has been made to solve the above-described conventional problems, and an object thereof is to provide a positive electrode active material and a lithium ion secondary battery that exhibit good cycle characteristics and have good output characteristics. .

The positive electrode active material according to the present invention includes a positive electrode active material, a coating layer covering the surface of the positive electrode active material, an element diffusion layer at the interface between the coating layer and the positive electrode active material, and the coating layer is a lithium-containing polyanion M _{1 + x} O _{4 + 3x} ^−y (M = V, Cr, Nb, Mo, W, x = 0, 1, y = 2, with a structure in which a polyanion skeleton forming a polyanion compound is centered on a transition metal. A positive electrode active material comprising the composition of 3 or 4).

    According to the present invention, since the polyanion compound is electrochemically stable, when the surface of the positive electrode active material is coated with the polyanion compound, the reaction with the electrolyte under a high potential can be suppressed and the cycle characteristics can be improved.

    According to the above invention, the transition metal element in the polyanion skeleton contained in the coating layer can easily form an element diffusion layer at the interface with the positive electrode active material, so that contact with the electrolyte can be effectively suppressed. The cycle characteristics can be effectively improved.

    Furthermore, according to the said invention, a high output characteristic can also be brought out simultaneously by the interface of the high electron conductivity obtained by the solid transition of the expensive transition metal contained in the polyanion skeleton in the positive electrode active material.

In the present invention, the polyanion skeleton includes chromate ion (CrO ₄ ²⁻ ), dichromate ion (Cr ₂ O ₇ ²⁻ ), orthovanadate ion (VO ₄ ³⁻ ), pyrovanadate ion (V ₂ O ₇ ⁴⁻ ), niobate ion (NbO ₄ ³⁻ ), molybdate ion (MoO ₄ ²⁻ ), and tungstate ion (WO ₄ ²⁻ ) are preferable.

    This is because the polyanion skeleton is composed of pentavalent or higher-valent transition metal ions, and such an expensive number of transition metals is drastically improved by being doped in a small amount at the Li ion site of the positive electrode active material.

    The positive electrode active material of the present invention is preferably a lithium-containing layered oxide.

    The lithium-containing layered oxide system is unstable at high potentials, and the capacity can be increased by further increasing the charging potential, so the present invention can effectively increase the energy density of the positive electrode active material. It is because it becomes.

    Furthermore, the element diffusion layer formed at the interface between the positive electrode active material and the coating layer of the present invention preferably has a thickness of 2 to 40 nm.

    By forming an element diffusion layer at the contact interface between the positive electrode active material and the polyanion compound, particularly in a thickness of 2 to 40 nm, the adhesion between the coating layer and the positive electrode active material is improved, and the positive electrode active material having more excellent cycle characteristics This is because the material can be used.

    Since the element diffusion layer has high electron conductivity, a positive electrode active material having more excellent output characteristics can be obtained.

The lithium-containing layered oxide contained in the positive electrode active material of the present invention has the following composition formula (1):
LiNi _a Co _b Mn _c Al _d O ₂ (1)
[In the above formula (1), 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c <0.34, 0 ≦ d ≦ 0.05]
It is desirable that

    This is because lithium-containing layered composite oxides containing a large amount of Ni, Co, and Mn have a particularly high capacity and can be used as a positive electrode active material having a high energy density.

    In addition, when the transition metal element contained in these positive electrode active materials is partially substituted with Al, the thermal stability and cycle characteristics are improved, so that a positive electrode active material having better safety and cycle characteristics can be obtained. .

    ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material and lithium ion secondary battery which show a favorable cycling characteristic and have a favorable output characteristic can be provided.

It is a schematic sectional drawing which shows an example of the lithium ion secondary battery of this invention. It is the schematic of the positive electrode active material of this invention. The method of estimating the element diffusion layer of the positive electrode active material of the present invention using a TEM-EDX line scan will be described.

    Hereinafter, the lithium ion secondary battery of the present invention will be described in detail.

A. Lithium Ion Secondary Battery FIG. 1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery of this embodiment. As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-shaped separator 18, an electrolyte containing lithium ions, a case 50 for containing these in a sealed state, and one end electrically connected to the negative electrode 20 and the other The negative electrode lead 62 whose one end protrudes outside the case, and the positive electrode lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case.

  The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

    The positive electrode active material layer 14 in FIG. 1 contains a positive electrode active material 140, and the positive electrode active material is made of a lithium-containing polyanion compound containing a polyanion skeleton centered on a pentavalent or higher transition metal. A layer 143, and an element diffusion layer 142 between the coating layer 143 and the positive electrode active material 141.

    In the present embodiment, the positive electrode active material 141 is coated with a lithium-containing polyanion compound containing a polyanion skeleton centered on a pentavalent or higher transition metal, and the coating layer 143 is electrochemically stable. The decomposition reaction that normally occurs at the contact interface between the organic electrolyte and the positive electrode active material can be suppressed. Therefore, it is possible to improve the decrease in cycle capacity during high voltage charging.

    Furthermore, the polyanion skeleton contained in the polyanion compound contains a high number of transition metal ions. If this transition metal ion has an ionic radius close to that of the transition metal ion contained in the positive electrode active material, it is possible to form an interface with high adhesion by element diffusion. As a result, contact with the electrolyte can be prevented, so that deterioration of cycle characteristics during high-voltage charging can be more effectively improved as compared with polyanion compounds such as lithium borate and lithium phosphate. At this time, the transition metal ion contained in the polyanion skeleton is not particularly limited as long as it forms a solid solution with the positive electrode active material.

    Further, the element diffusion layer 142 has high electron conductivity. This is considered to be caused by the fact that holes are injected as carriers by doping a transition metal site having a valence of 3 to 4 with a transition metal element having a valence of 5 or more. Since the lithium-containing polyanion compound usually has lithium ion conductivity, a positive electrode active material excellent in output characteristics can be obtained together with high electronic conductivity.

Therefore, in the positive electrode active material 140 of the present embodiment, cycle characteristics and output characteristics during high voltage charging can be improved at the same time.
Hereinafter, the lithium ion secondary battery of this invention is demonstrated for every structure.

1. The positive electrode has a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12, and the positive electrode active material layer 14 is a polyanion skeleton centered on a pentavalent or higher valent transition metal. And a positive electrode active material 140 having a coating layer 143 made of a lithium ion-containing polyanion compound. Further, an element diffusion layer is formed at the interface between the coating layer 143 and the positive electrode active material 140. Hereinafter, each structure of the positive electrode will be described.

(Positive electrode active material)
The positive electrode active material is not particularly limited as long as it can occlude and release lithium ions. For example, LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O which are spinel positive electrode active materials. ₄ , LiMnO ₂ which is a layered oxide system, LiCoO ₂ , LiNiO ₂ , LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , olivine-based positive electrode LiFePO ₄ as an active material, LiCoPO _4, can be cited LiNiPO _4, etc., particularly preferably a positive electrode active material of a lithium-containing layered oxide.

    Since the layered oxide-based positive electrode active material increases the Li insertion / desorption amount (charge / discharge capacity) by increasing the charging voltage, the energy density is improved, and the reaction with the electrolyte easily occurs. This is because the improvement of the output characteristics becomes more effective.

    The particle diameter of the positive electrode active material 141 is preferably about 30 nm to 10 μm as the primary particle diameter. If it is smaller than this, it is difficult to form a uniform coating layer, and if it is too large, the output characteristics tend to be lowered. The primary particles may be granulated to form secondary particles, in which case a secondary particle size of about 5 μm to 50 μm is preferred. If the particle size is smaller than this, the coatability is deteriorated, and if it is too large, not only the output characteristics are deteriorated but also it is difficult to obtain a uniform coating film.

    As content of the positive electrode active material contained in the positive electrode active material layer 14, although it changes also with kinds of positive electrode active material, it is preferable that it is specifically about 80 wt%-99 wt%, More preferably, it is 90 wt%- It is in the range of 99 wt%.

(Coating layer)
The coating layer 143 is made of a crystalline or amorphous lithium-containing polyanion compound, and is particularly characterized in that the polyanion skeleton constituting the polyanion compound has a structure centered on a transition metal.

Such a lithium-containing polyanion compound is represented by the general formula Li _y M _{1 + x} O _{4 + 3x} (x = 0, 1, y = 2, 3, 4, M = transition metal ion having a valence of 5 or more), and transitions in the structure It contains MO ₄ tetrahedron centered on metal, or M ₂ O ₇ which is a dimerized structure of the tetrahedron by sharing apexes as a polyanion, and the crystal structure is formed by surrounding the polyanion with lithium ions. I'm making it. Since these lithium ions have abundant movement paths and can diffuse in the solid, the polyanion compound can function as a lithium ion conductor.

    Further, the metal-oxygen bond forming the polyanion has a strong covalent bond and high chemical stability. Since it does not easily react with the electrolyte even at a high potential, a reaction suppression layer that suppresses the reaction between the positive electrode active material and the electrolyte can be obtained by covering the surface of the positive electrode active material.

    The coating layer can be formed by a dry method such as mechanical milling or a wet method such as sol-gel coating, and desirably coats the entire positive electrode active material uniformly, but it may be non-uniform. Further, the thickness of the coating layer is not particularly limited.

    The lithium-containing polyanion compound may be crystalline or amorphous.

    When the lithium-containing polyanion is crystalline, identification by X-ray diffraction measurement (XRD) is possible.

    When the lithium-containing polyanion is amorphous, identification by infrared spectroscopy (IR), Raman spectroscopy, or inductively coupled plasma (ICP) is possible.

(Element diffusion layer)
The element diffusion layer 142 is formed by coating the surface of the positive electrode active material 141 with a lithium-containing polyanion compound and then performing a heat treatment. A part of the transition metal element contained in the polyanion skeleton diffuses to the positive electrode active material side. In addition, the transition metal element in the positive electrode active material is partially diffused to the polyanion compound side. Since this element diffusion layer 142 is based on thermal diffusion of elements, it can be formed in principle regardless of whether the coating layer is crystalline or amorphous.

    The element diffusion layer 142 is formed at the interface between the positive electrode active material and the coating layer (FIG. 2). Even if the coating layer 143 is not uniform, the element diffusion layer can be formed as long as the coating layer is in contact with the positive electrode active material, so that the effects of the coating layer and the element diffusion layer can be obtained. .

    The thickness of the element diffusion layer 142 can be controlled by factors that govern the element diffusion rate, such as heat treatment conditions.

    The thickness of the element diffusion layer 142 is preferably 1 nm to 100 nm, more preferably about 1 nm to 50 nm. If the thickness of the element diffusion layer is too thick, the charge / discharge capacity decreases. This is presumably because the solid solution of a high number of transition metal elements that do not contribute to charge and discharge into the positive electrode active material becomes significant.

    The thickness of the element diffusion layer 142 is determined by energy dispersive X-ray analysis combined with a scanning transmission electron microscope, electron beam energy loss spectroscopy (TEM-EDX or STEM-EELS), or flight secondary ion mass spectrometry (TOF- SIMS). The thickness of the element diffusion layer 142 is preferably calculated by taking an average of 10 or more points.

(Other)
The positive electrode 10 used in the present embodiment may further contain a conductive agent and a binder (binder).
Examples of the binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
Examples of the conductive agent include carbon black such as acetylene black and ketjen black.

    As a method for forming the positive electrode active material layer 14, a general method can be used. For example, after the positive electrode active material having the above-mentioned solid powder on the surface, a binder, and a positive electrode active material layer forming paste containing a conductive agent are applied on the positive electrode current collector 12 and dried, The positive electrode active material layer 14 can be formed by pressing.

    The positive electrode active material layer 14 may be formed on the positive electrode current collector 12. The material of the positive electrode current collector 12 is not particularly limited as long as it has conductivity, and examples thereof include aluminum, SUS, nickel, iron, and titanium. Of these, aluminum and SUS are preferably used.

2. Negative Electrode The negative electrode 20 used in the present invention has a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22, and if necessary, a conductive agent and a binder. An agent may be contained.

    The negative electrode active material layer 24 contains at least a negative electrode active material, and may contain a conductive agent and a binder as necessary.

The negative electrode active material is not particularly limited as long as it can occlude and release lithium ions. For example, metal lithium, lithium alloy, metal oxide, metal sulfide, metal nitride, and graphite And carbon materials. The negative electrode active material may be in the form of a powder or a thin film.
In addition, about the binder and electrically conductive agent used for a negative electrode active material layer, the thing similar to the binder and electrically conductive agent used for the said positive electrode active material layer can be used.
Moreover, since the formation method of a negative electrode active material layer is the same as the formation method of the said positive electrode active material layer, description here is abbreviate | omitted.

    The negative electrode active material layer 24 may be formed on the negative electrode current collector 22. The material of the negative electrode current collector 22 is not particularly limited as long as it has conductivity, and examples thereof include copper, stainless steel, and nickel.

3. Separator The separator 18 is not particularly limited as long as it has a function of retaining an electrolyte, and examples thereof include a porous film such as polyethylene and polypropylene, a resin nonwoven fabric, and a glass fiber nonwoven fabric.

4). Electrolyte The electrolyte used in the present embodiment is disposed so as to be in contact with the positive electrode active material having a coating layer made of the lithium-containing polyanion compound, and may be organic or inorganic, but is particularly an organic electrolyte. High effect in case.

    The organic electrolyte is not particularly limited as long as it can be disposed so as to be in contact with the positive electrode active material having a coating layer made of the lithium-containing polyanion compound and can be oxidatively decomposed. Examples thereof include an electrolytic solution, a polymer electrolyte, and a gel electrolyte.

As the organic electrolyte, a non-aqueous electrolyte containing a lithium salt and a non-aqueous solvent is usually used.
The lithium salt is not particularly limited as long as it is a lithium salt used in a general lithium ion secondary battery. For example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , Examples include LiC (CF ₃ SO ₂ ) ₃ and LiClO ₄ .

The non-aqueous solvent is not particularly limited as long as it can dissolve the lithium salt. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane. 1,2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. It is done. These nonaqueous solvents may be used alone or in combination of two or more. Moreover, a low temperature molten salt can also be used as a non-aqueous electrolyte solution.

The polymer electrolyte contains a lithium salt and a polymer.
As a lithium salt, the thing similar to the lithium salt used for the said organic electrolyte solution can be used.
The polymer is not particularly limited as long as it can be gelled. For example, polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride (PVDF), polyurethane, polyacrylate, cellulose Etc.

5. Battery Case The case 50 is for sealing the laminate 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

B. Manufacturing method of lithium ion secondary battery The manufacturing method of the lithium ion secondary battery 100 of this invention is the manufacturing method of the lithium ion secondary battery 100 which has the positive electrode active material layer 14, the negative electrode active material layer 24, and an organic electrolyte. A cathode active material preparation for preparing a cathode active material having a coating layer made of a lithium-containing polyanion compound on the surface by mixing lithium-containing polyanion compound particles for coating a cathode active material and a cathode active material Process.

According to this embodiment, a positive electrode active material having a coating layer made of a lithium-containing polyanion compound can be obtained. By forming a positive electrode active material layer using such a positive electrode active material, it is possible to suppress a decrease in cycle capacity during high-voltage charging and to suppress a decrease in capacity during rapid charge / discharge. A lithium ion secondary battery can be obtained.
Hereinafter, each process in the manufacturing method of the lithium secondary ion battery of this embodiment is demonstrated.

(1) Cathode Active Material Material Preparation Step In the embodiment, the cathode active material material preparation step is a mixture of lithium-containing polyanion compound particles and cathode active material particles, and a cathode active material having a coating layer made of a lithium-containing polyanion compound. This is a process for preparing a material.

    The method for mixing the lithium-containing polyanion compound particles and the positive electrode active material is not particularly limited as long as it is a method capable of obtaining a positive electrode active material having a coating layer made of a lithium-containing polyanion compound. Further, a mechanical milling method may be used as a mixing method.

    The atmosphere when mixing the lithium-containing polyanion compound particles and the positive electrode active material particles may be, for example, an air atmosphere or an inert gas atmosphere.

(2) Element diffusion layer formation process Next, the element diffusion layer formation process in this embodiment is demonstrated. In the element diffusion layer forming step in the present embodiment, a positive electrode active material coated with a lithium-containing polyanion compound containing a polyanion skeleton centered on a pentavalent or higher transition metal is heat-treated, and the positive electrode active material, the lithium-containing polyanion compound, This is a step of forming an element diffusion layer in which the transition metal ions are diffused in the vicinity of the interface.

As heat processing temperature in this process, it is preferable to exist in the range of 400 to 900 degreeC, and it is more preferable to exist in the range of 500 to 800 degreeC.
Further, the heat treatment time in this step is preferably in the range of 0.5 hours to 3 hours, and more preferably in the range of 0.5 hours to 1.5 hours.

    The heat treatment atmosphere in this step is not particularly limited as long as the target element diffusion layer can be formed and the cathode active material material is not deteriorated. For example, an air atmosphere, an oxygen atmosphere, a nitrogen atmosphere And an argon-hydrogen atmosphere. Examples of the heat treatment method for the positive electrode active material include a method using a firing furnace.

    The element diffusion layer forming method in the present embodiment is not limited to heat treatment as long as the element diffusion layer can be formed. As long as the element diffusion layer can be formed, irradiation with electromagnetic waves (infrared rays or microwaves), mechanical milling, or the like may be used.

    Since the positive electrode active material is described in “A. Lithium secondary battery”, description thereof is omitted here.

(3) Other process The manufacturing method of the lithium ion secondary battery 100 of this embodiment will not be specifically limited if it has at least the said positive electrode active material material preparation process, For example, the said positive electrode active material material A positive electrode active material layer forming step for forming a positive electrode active material layer 14 containing a positive electrode active material having a coating layer made of a lithium-containing polyanion compound obtained in the preparation step, and a negative electrode active material for forming a negative electrode active material layer 24 You may have the material layer formation process, the battery assembly process of assembling a lithium ion secondary battery, etc. Since these steps are the same as those in a general lithium secondary battery, description thereof is omitted here. Further, the obtained lithium ion secondary battery is the same as that described in the above “A. Lithium secondary battery”, and thus the description thereof is omitted here.

    The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

    Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
(Preparation of positive electrode active material)
Li ₃ VO _{4 as} a coating material and LiN _i0.8 Co _{0.15 Al 0.05} O _{2 as} a positive electrode active material are weighed so as to be 1:99 (weight ratio), and mixed and combined by a pot mill. Processed. Subsequently, the positive electrode active material and the media were separated by sieving, and heat treatment was performed at 500 ° C. for 1 hour and a half in a muffle furnace to obtain a positive electrode active material.

(Preparation of positive electrode active material layer)
The above positive electrode active material and acetylene black were mixed, and a polyvinylidene fluoride (PVDF) binder dissolved in n-methylpyrrolidone (NMP) was added to prepare a slurry. The mixing ratio of the positive electrode active material, acetylene black, and PVDF was 92: 4: 4 (weight ratio). The slurry was applied to an Al current collector, dried, and pressed to prepare a positive electrode active material layer.

(Production of lithium ion secondary battery)
The positive electrode active material layer and the copper foil (thickness 16 μm) are cut into predetermined dimensions, respectively, and an aluminum foil (width 4 mm, length 40 mm, thickness 100 μm), nickel foil (width 4 mm, length) are used as external lead terminals, respectively. 40 mm, thickness 100 μm) was ultrasonically welded. Polypropylene (PP) was wrapped around this external lead terminal and thermally bonded. A polyethylene separator cut to a predetermined size is sandwiched between the positive electrode and copper foil, and these battery elements are thermocompression bonded to an aluminum laminate material composed of a polyethylene terephthalate (PET) layer, an Al layer, and a polypropylene (PP) layer. Fixed by. The separator used was a polyethylene microporous membrane. After the obtained battery element was dried, it was transferred into a glove box filled with argon gas, and a Li foil cut to a predetermined size was pasted on the copper foil. The thickness of the PET layer of the laminating agent was 12 μm, the thickness of the Al layer was 40 μm, and the thickness of the PP layer was 50 μm. Further, in the production of the battery outer package, the PP layer was disposed inside the outer package. A battery element is placed in the outer package, and a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 3: 7, and lithium lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / dm. ^An appropriate amount of the electrolyte solution contained in ³ was added, and the outer package was vacuum-sealed. Thereby, the lithium ion secondary battery of Example 1 was produced.

(Measurement of electrical characteristics)
The battery of Example 1 was charged at a constant current up to 4.5 V at a current value corresponding to 0.2 C as the current density, and then charged at a constant voltage at 4.5 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.02C. Thereafter, constant current discharge was performed to 2.5 V at a current density of 0.2C. At this time, the initial discharge capacity of Example 1 was 210 mAh / g. Next, the current density was increased to a current value corresponding to 2.0 C, charged at a constant current up to 4.5 V, and then charged at a constant voltage at 4.5 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.2C. Thereafter, constant current discharge was performed to 2.5 V at a current density of 2.0 C. The ratio of the discharge capacity at 2.0 C to the discharge capacity at 0.2 C obtained by these measurements is shown in Tables 1 to 3 as the discharge capacity ratio.
After the measurement of the above electric characteristics, the battery was further charged at a constant current up to 4.5 V at a current value corresponding to 1.0 C, and then charged at a constant voltage at 4.5 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.05C. Thereafter, constant current discharge was performed to 2.5 V at a current density of 1.0 C. A cycle test was repeated for 30 cycles. The test was conducted at 25 ° C. Assuming that the discharge capacity at the first cycle of the battery of Example 1 was 100%, the discharge capacity after 30 cycles was 88.8%. In this specification, when the discharge capacity at the first cycle is 100%, the ratio of the discharge capacity after 30 cycles is referred to as a capacity maintenance rate and is shown in Tables 1 to 3. A high cycle characteristic means that the discharge capacity after 30 cycles is maintained high, and indicates that the battery is excellent in durability in the charge / discharge cycle.
Table 1 shows the electrical characteristics along with the preparation conditions of the positive electrode active material.

[Comparative Example 1]
A lithium ion secondary battery was produced and its electrical characteristics were measured in the same manner as in Example 1 except that there was no mixing / combination process of the coating material and the positive electrode active material.
Table 1 shows the discharge capacity ratio and capacity retention rate. It can be seen that both the discharge capacity ratio and the capacity retention rate are lower than those having the coating material.

[Comparative Examples 2 and 3]
A lithium ion secondary battery was produced and its electrical characteristics were measured in the same manner as in Example 1 except that Li ₃ PO ₄ and Li ₃ BO ₃ were used as the covering materials, respectively.
Table 1 shows the discharge capacity ratio and capacity retention rate. It can be seen that both the discharge capacity ratio and the capacity retention rate are lower in the case of coating with Li ₃ PO ₄ or Li ₃ BO ₃ than in the case of containing a transition metal in the polyanion skeleton.

[Examples 2 to 5]
A lithium ion secondary battery was produced and its electrical characteristics were measured in the same manner as in Example 1 except that the coating material was the coating material shown in Table 1.
Table 1 shows the discharge capacity ratio and capacity retention rate. It can be seen that the higher the discharge capacity ratio, the higher the capacity retention rate. As the electrical characteristics, those coated with Li ₃ NbO ₄ are the most excellent, and the characteristics are good in the order of Li ₃ NbO ₄ > Li ₂ Cr ₂ O ₇ > Li ₂ CrO ₄ > Li ₃ VO ₄ > Li ₂ WO ₄ .

[Examples 6 to 8]
Regarding Li ₃ NbO ₄ which is the coating material having the best characteristics in Examples 1 to 5, the electrical characteristics were examined by changing the type of the positive electrode active material.
Table 2 shows the types of positive electrode active materials used. Except for the type of active material, synthesis of a positive electrode active material, production of a lithium ion secondary battery, and evaluation of electrical characteristics were performed in the same manner as in Example 4.

[Comparative Examples 4 to 6]
In order to verify the effect on the electrical characteristics of Li ₃ NbO _4, the electrical characteristics were evaluated without using a coating material for the positive electrode active materials used in Examples 6-8.
A lithium ion secondary battery was prepared and electrical characteristics were evaluated in the same manner as in Comparative Example 1 except that the type of the positive electrode active material was as shown in Table 2.
As shown in Table 2, it can be seen that when the coating is performed with Li ₃ NbO ₄ , both the discharge capacity ratio and the capacity retention ratio are improved. The positive electrode active material having the best characteristics was obtained by coating LiCoO ₂ with Li ₃ NbO ₄ , and the discharge capacity ratio and capacity retention rate were 0.82 and 97.1%, respectively.

[Examples 9 to 12 and Comparative Examples 7 to 8]
In order to investigate the influence of the element diffusion layer generated at the interface between the coating layer and the positive electrode active material, the electrical characteristics of LiCoO ₂ coated with Li ₃ NbO ₄ were examined by changing the heat treatment temperature.
Except that the heat treatment temperature is as shown in Table 3, synthesis of a positive electrode active material, production of a lithium ion secondary battery, and evaluation of electrical characteristics were performed in the same manner as in Example 6.

    FIG. 3 is an explanatory diagram of a method for measuring the thickness of the element diffusion layer in the present embodiment, using the element distribution ratio obtained by TEM-EDX line scan in the vicinity of the element diffusion layer. In this embodiment, the ratio of the transition metal between the coating layer and the positive electrode active material is measured by TEM-EDX line scan in the thickness direction of the coating layer, and the transition metal and the positive electrode active material transition metal constituting the coating layer are configured. The thickness of the element diffusion layer was measured by estimating the range in which both transition metals were detected. At this time, the element having the smaller abundance ratio was regarded as having element diffusion from the point where it was 5% or more (or less), and the average thickness of the element diffusion layer was determined based on the measured values at 10 points.

    Table 3 shows the thickness (10-point average value) of the element diffusion layers. When the thickness of the element diffusion layer is between 2.5 nm and 40.1 nm, the effect of improving the discharge capacity ratio and the capacity retention rate is particularly high. It can be seen that the discharge capacity ratio decreases when there is no element diffusion layer. It can also be seen that when the heat treatment temperature is 900 ° C., the discharge capacity ratio and the capacity retention rate tend to decrease.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode 20 ... Negative electrode 12 ... Positive electrode collector 14 ... Positive electrode active material layer 140 ... Positive electrode active material material 141 ... Positive electrode active material 142 ... Element diffusion layer 143. ..Coating layer 18 ... Separator 22 ... Negative electrode current collector 24 ... Negative electrode active material layer 30 ... Battery element 50 ... Case 52 ... Metal foil 54 ... Polymer film 60 62 ... Lead 100 ... Lithium ion secondary battery.

Claims (6)

A positive electrode active material;
A coating layer covering the surface of the positive electrode active material;
An element diffusion layer at the interface between the coating layer and the positive electrode active material;
The coating layer is made of a lithium-containing polyanion compound, and the polyanion skeleton forming the polyanion compound has a structure centered on a transition metal, and M _{1 + x} O _{4 + 3x} ^−y (M = V, Cr, Nb, Mo, W, x = 0) 1, y = 2, 3, 4) A positive electrode active material having a composition. The polyanion skeleton is
Chromate ion (CrO ₄ ²⁻ ), dichromate ion (Cr ₂ O ₇ ²⁻ ), orthovanadate ion (VO ₄ ³⁻ ), pyrovanadate ion (V ₂ O ₇ ⁴⁻ ), niobate ion ( 2. The positive electrode active material according to claim 1, wherein the positive electrode active material is any one or more of NbO ₄ ³⁻ ), molybdate ions (MoO ₄ ²⁻ ), and tungstate ions (WO ₄ ²⁻ ).   The positive electrode active material according to claim 1, wherein the positive electrode active material is a lithium-containing layered oxide.   The positive electrode active material according to claim 1, wherein the element diffusion layer has a thickness of 2 to 40 nm. The lithium-containing layered oxide has the following composition formula (1)
LiNi _a Co _b Mn _c Al _d O ₂ (1)
[In the above formula (1), 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c <0.34, 0 ≦ d ≦ 0.05]
The positive electrode active material according to claim 3, wherein the positive electrode active material is expressed by: A positive electrode having a positive electrode current collector and a positive electrode active material layer including a positive electrode active material;
A negative electrode having a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material;
A separator located between the positive electrode and the negative electrode;
An electrolyte in contact with the negative electrode, the positive electrode, and the separator;
A lithium ion secondary battery, wherein the positive electrode active material is the positive electrode active material according to claim 1.

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